Titanium dioxide (TiO2) is manufactured around the world for use in a variety of applications. For example, titanium dioxide is commonly used as a pigment in polymer compositions, paint formulations, paper products, and other products.
Titanium dioxide pigments are generally produced in powder form with certain properties and performance characteristics (for example, a certain particle size, shape and lattice structure and certain rheological properties when dispersed in polymer compositions). The pigment particles are typically coated with one or more materials to modify or enhance the properties and characteristics of the pigment for particular applications. For example, the pigment particles are often coated with compounds that function to improve the opacity, brightness, durability and rheological properties of the pigment.
Titanium dioxide pigments can be manufactured by either the sulfate process or the chloride process. In some cases, the particular manufacturing method utilized can impact the properties and characteristics of the pigments and the end application associated therewith.
In the sulfate process for manufacturing titanium dioxide, a titanium slag ore, usually an ilmenite, is dissolved in sulfuric acid to form a mixture of sulfates, including titanyl sulfate. Iron is removed from the solution. The titanyl sulfate is then hydrolyzed in solution to yield insoluble, hydrated titanium dioxide. The hydrated titanium dioxide is heated in a calciner to evaporate the water and decompose the sulfuric acid in the solid. The solid is then converted into titanium dioxide seed crystals which can be milled to the desired size.
In the chloride process for manufacturing titanium dioxide, a dry titanium dioxide ore is fed into a chlorinator together with coke and chlorine to make a titanium halide (such as titanium tetrachloride). Streams of gaseous titanium halide (such as titanium tetrachloride) and oxygen are heated and introduced at high flow rates into an elongated vapor phase oxidation reactor conduit. A high temperature (approximately 2000° F. to 2800° F.) oxidation reaction takes place in the reactor conduit whereby particulate solid titanium dioxide and gaseous reaction products are produced. The titanium dioxide and gaseous reaction products are then cooled, and the titanium dioxide particles are recovered.
In a typical titanium dioxide production process, the titanium dioxide is produced in the form of a fine powder, which tends to be very fluffy. The fine particle size and fluffiness of the produced titanium dioxide powder can cause problematic dust to form when the pigment is transported, bagged and unloaded from bags. Additionally, the fine particle size of the titanium dioxide product can result in significant bridging of the pigment particles which can make it difficult to convey the product and prohibit the product from easily flowing through hoppers and other transfer equipment subsequently used to incorporate the titanium dioxide into other products.
In order to reduce the problematic dust and improve the flowability of the pigment, the titanium dioxide powder can be formed into agglomerates. Unfortunately, however, agglomeration of the powder can decrease important performance characteristics of the pigment in end applications thereof. For example, the fine particle size of the titanium dioxide is very beneficial with respect to performance characteristics of the titanium dioxide such as the ability of the titanium dioxide to be dispersed in polymer compositions and the rheological characteristics of the pigment in such compositions. Thus, efforts to reduce the dustiness and improve pigment flow characteristics of the titanium dioxide tend to come at a sacrifice to important performance characteristics of the titanium dioxide.